Metal to metal adhesives and techniques of strongly joining metal to synthetic resin are required in a wide range of industrial fields beyond component manufacturing of automobiles, household appliances, industrial machines and the like, and many adhesives have been developed for this reason. That is, techniques of adhesion and joining are basic and applied key technology in all manufacturing industries.
Heretofore, joining methods without using adhesives have also been studied. Among them, it was “NMT (abbreviation of nano-molding technology)” that was developed by the present inventors and had a great impact on manufacturing industries. The NMT is a technique of joining an aluminum alloy with a resin composition (hereinafter, abbreviated as “injection joining”) where molten engineering resin is injected onto an aluminum alloy part that is previously inserted in an injection mold, so that a resin part is molded while the molded product is joined to the aluminum alloy component at the same time. Patent literature 1 discloses a technique of injection-joining a polybutylene terephthalate resin (hereinafter referred to as “PBT”) to a shaped aluminum alloy with a specific surface treatment. Patent literature 2 discloses a technique of injection-joining a polyphenylene sulfide resin (hereinafter referred to as “PPS”) to an aluminum alloy with a specific surface treatment. In the following, the principle of the injection joining in patent literatures 1 and 2 will be briefly described.
(NMT)
The NMT requires two conditions for the aluminum alloy and one condition for the resin composition. The two conditions of the aluminum alloy are described below.
(1) The surface of the aluminum alloy is covered with ultrafine asperities of 20 to 80 nm period or ultrafine recesses or ultrafine protrusions of 20 to 80 nm diameter. As an indicator, it is preferable to be covered with ultrafine asperities having an RSm of 20 to 80 nm. It is also preferable to be covered with ultrafine recesses or ultrafine protrusions having an Rz of 20 to 80 nm. Furthermore, it is also preferable to be covered with ultrafine asperities having an RSm of 20 to 80 nm and an Rz of 20 to 80 nm. RSm represents the mean width of profile elements defined by Japan Industrial Standards (JIS B0601-2001, ISO 4287-1997), and Rz represents the maximum height of a profile defined by Japan Industrial Standards (JIS B0601-2001, ISO 4287-1997).
The aluminum alloy has a surface layer of an aluminum oxide film, having a thickness of 3 nm or more.
(2) Ammonia, hydrazine or water-soluble amine compound is chemically adsorbed on the surface of the aluminum alloy.
On the other hand, the condition of the resin composition is as follows.
(3) The main component is rigid crystalline thermoplastic resin that is capable of reacting with amine compounds in a broad sense at 150° C. to 200° C. such as ammonia, hydrazine and water-soluble amines. Specifically, the resin composition contains PBT, PPS, polyamide resin or the like as the main component.
When the resin composition contained PBT or PPS as the main component (i.e. satisfied the condition of (3)) as well as 10 to 40 mass % of a glass fiber, it exhibited unprecedentedly strong joining strength with an aluminum alloy that satisfied the conditions of (1) and (2). In the condition where the aluminum alloy and resin composition were both plate-shaped and joined to each other in a certain area (0.5 cm2), the shear fracture was 20 to 25 MPa.
To achieve stronger joining strength by the NMT, one more condition is further added for the resin composition.
(4) A different polymer from the main component polymer is contained, and a majority of the different polymer is mixed with the base crystalline thermoplastic resin at the molecular level.
The purpose of adding this condition (4) is to decrease the crystallization rate when molten resin composition is rapidly cooled. This is based on an idea that if a different polymer is mixed at the molecular level, it inhibits the arrangement during crystallization from the molten state, which eventually leads decreasing the crystallization rate in rapid cooling. It was presumed that this enables the resin composition to sufficiently penetrate into the ultrafine asperities before solidified, which contributes to improving the joining strength. This presumption turned out to be true as a result.
When the resin composition contained PBT or PPS as the main component (i.e. satisfied the condition of (3)), satisfied the condition of (4) (was compounded with a different polymer) and further contained 10 to 40 mass % of a glass fiber, it exhibited very strong joining strength with aluminum alloy that satisfied the conditions of (1) and (2). In the condition where the aluminum alloy and resin composition were both plate-shaped and joined to each other in a certain area (0.5 to 0.8 cm2), the shear fracture was 25 to 30 MPa. In the case of a resin composition where different polyamides were compounded, the shear fracture was 20 to 30 MPa.
(New NMT)
With respect to metal alloys besides the aluminum alloy, the present inventors also found the conditions in which such metal alloys can strongly join with thermoplastic resin such as PBT or PPS by injection joining as described in patent literatures 3, 4, 5, 6 and 7. The mechanism of the injection joining in these conditions was named “new NMT”. All of these inventions were made by the present inventors. The required conditions of this more widely applicable “new NMT” will be described below. There are conditions for both metal alloy and injection resin. Firstly, the following three conditions ((a), (b) and (c)) are required for the metal alloys.
(a) The first condition is that the metal alloys have such a rough surface by chemical etching that the asperities have a period of 1 to 10 μm and a vertical interval up to approximately a half of the period, i.e. 0.5 to 5 μm. It is however difficult to precisely cover the entire surface with such rough surface by means of a nonuniform and variable chemical reaction. Specifically, it is therefore required that when measured by a roughness meter, the asperities have such a roughness profile as an irregular period in a range of 0.2 to 20 μm and a vertical interval in a range of 0.2 to 5 μm. Alternatively, when the metal alloy surface is scanned with a dynamic mode scanning probe microscope of the latest model, it is deemed that the above condition is substantially satisfied if the surface has such a roughness as an RSm of 0.8 to 10 μm and Rz of 0.2 to 5 μm. Since an idealistic rough surface has an asperity period of approximately 1 to 10 μm as described above, the present inventors named such surfaces “micron-order rough surface” in plain words.
(b) The second condition is that ultrafine asperities having a period of 5 nm or more are further formed on the micron-order rough surface of the metal alloy. In other words, it is required to be an asperate surface through micron-order eyes. In order to satisfy this condition, the above metal alloy surface is subjected to fine etching to form the ultrafine asperities on inner walls of the micron-order rough recesses. The ultrafine asperities have a period of 5 to 500 nm, preferably 10 to 300 nm, more preferably 30 to 100 nm (the optimum value is 50 to 70 nm).
Describing these ultrafine asperities, if the asperity period is less than 10 nm, the resin component clearly has difficulty in penetrating into them. Further, since the vertical interval normally becomes low in such cases, they are considered as a smooth surface for the resin. As a result, they do not function as spikes. If the period is approximately 300 to 500 nm or more (in this case, the micron-order rough recesses are assumed to have a diameter or period of nearly 10 μm), they become less effective since the number of spikes in each micron-order rough recess is drastically decreased. It is thus required that the ultrafine asperities have a period ranging from 10 to 300 nm in principle. However, depending on the shape of the ultrafine asperities, the resin may penetrate into the gaps even if the period is 5 to 10 nm. For example, tangled rod-like crystals having a diameter of 5 to 10 nm fall under the case. Also, even if the period is 300 to 500 nm, the ultrafine asperities of some shapes tend to have anchoring effect. For example, a shape like pearlite structure, which is composed of infinitely continuous steps having a height and depth of tens to 500 nm and a width of hundreds to thousands nm, falls under the case. Including these cases, the required period of the ultrafine asperities is specified to 5 to 500 nm.
With respect to the above first condition, the ranges of the RSm and Rz are conventionally specified to 1 to 10 μm and 0.5 to 5 μm respectively. However, even when the RSm and Rz respectively fall within the ranges of 0.8 to 1 μm and 0.2 to 0.5 μm, the joining strength is retained strong as long as the asperity period of the ultrafine asperities is within a particularly preferable range (approximately 30 to 100 nm). Hence, the range of the RSm was extended lower to a certain extent. Specifically, the RSm and Rz were respectively specified to the ranges of 0.8 to 10 μm and 0.2 to 5 μm.
(c) Furthermore, the third condition is that the metal alloy has a ceramic surface layer. Specifically, as for originally anticorrosive metal alloys, the surface layer is required to be a metal oxide layer having a thickness equal to or more than their natural oxide layer. As for metal alloys having relatively low corrosion resistance (e.g. magnesium alloy, general steels, and the like), the third condition is that the surface layer is metal oxide or metallic phosphate film that is produced by chemical conversion or the like.
On the other hand, the conditions for the resin are described below.
(d) The resin is rigid crystalline thermoplastic resin. Specifically, the resin composition contains PBT, PPS, polyamide resin or the like as a main component.
Furthermore, to achieve strong joining strength, the new NMT requires one more additional condition for the resin composition.
(e) A different polymer from the main component polymer is contained, and a majority of the different polymer is mixed with the base crystalline thermoplastic resin at the molecular level.
The above conditions (d) and (e) are the same as conditions (3) and (4) of the NMT. That is, the optimum injection resin is PBT resin, PPS resin or polyamide resin that is compounded with a different polymer. These resin compositions start generating initial seed crystals late when they are injected to a mold by an injection molding machine and cooled rapidly in the mold to be crystallized and solidified. By means of this property, an attempt was made to make the injection resin reach the bottoms of the micron-order rough recesses. It was presumed that the heads of the flowing resin also penetrated to the recesses of the ultrafine asperities of 5 to 500 nm period that were present on the inner wall of these recesses, and then crystallized and solidified in the state of, so to say, sticking the heads. In practice, when the above resin was injected to different metal alloys that were surface-treated so as to fulfill conditions (a), (b) and (c), the resin was penetrated in the ultrafine asperities, which greatly contributed to the joining strength.
Plate-shaped magnesium alloy, aluminum alloy, copper alloy, titanium alloy, stainless steel, general steel and the like were processed so that their surfaces satisfy conditions (a), (b) and (c). PBT resin or PPS resin was injection-molded into a plate shape on the surfaces. Plate-to-plate joint products were thus obtained. In the condition where these metal alloys and resin compositions are both plate-shaped and they are joined to each other in a certain area (about 0.5 to 0.8 cm2), the shear fracture strengths were 25 to 30 MPa. In these cases, the fracture was caused by destruction at the side of molded resins. Since the new NMT provided very high joining strength and the fracture was thus caused by the destruction at the side of the resin, the joining strengths were on the same level among different metal alloys (patent literatures 3 to 7).
Patent literature 1: WO 03/064150A1 (aluminum alloy)
Patent literature 2: WO 2004/041532A1 (aluminum alloy)
Patent literature 3: WO 2008/069252A1 (magnesium alloy)
Patent literature 4: WO 2008/047811A1 (copper alloy)
Patent literature 5: WO 2008/078714A1 (titanium alloy)
Patent literature 6: WO 2008/081933A1 (stainless steel)
Patent literature 7: WO 2009/011398A1 (general steel)